The invention relates generally to the field of fluid driven turbines, such as wind turbines, wherein electrical power is generated from airflow across rotor blades. More particularly, the invention relates to such turbines wherein the blades extend from a central rotating hub or shaft member to an annular rotating ring or rim member. Even more particularly, the invention relates to mechanisms and methods for adjusting the pitch of the blades to maximize efficiency.
Wind turbines, also known as windmills, wind generators, wind machines or the like, are well known devices for producing energy, typically electrical energy, by harnessing the power of wind. Typical wind turbines are very large structures, standing hundreds of feet tall and having rotor blades extending hundreds of feet, the rotor blades being mounted to a central hub or shaft. Large elongated blades are required since the area of blade sweep is proportional to the power that can be produced by a given wind turbine. This size creates a problem, in that each blade must act as a cantilever that can support the elongated blade and the blades must be increasingly massive toward the central hub in order to preclude the blade from breaking during use. Even with the use of high tech composites, carbon fibers and the like, a practical limit has been reached. For example, most large wind turbines have three blades, as this has been found to be the most efficient design. The desire to increase blade length in order to capture more wind is offset by the accompanying reduction in shaft revolutions per minute (rpm). Reduced rpm's require larger reduction gearboxes, which is undesirable. Decreasing blade pitch in order to increase the rpm's reduces torque, which is undesirable. In addition, repair or replacement of these giant blades is difficult and can be extremely expensive. The cost of upkeep and repair reduces the overall benefits of energy production using wind technology. Thus, known wind turbine designs always have to make sacrifices in order to balance these competing factors.
An alternative type of wind turbine incorporates an outer, annular ring or rim that is joined to the blades, such that the blade tips are no longer self-supporting and thereby allowing the blades to be lighter and/or longer, easier to manufacture and repair, etc. The blades extend from the central rotating hub or shaft to the annular rim and are therefore supported at both ends. In this embodiment, it is also known to utilize the rotational energy from the rotating annular rim rather than the central hub, since the annular rim travels at much faster speed. Rollers, wheels, gears, magnets or the like are used to transfer the rotational energy from the rim to one or more generator mechanisms or the like. Examples of such turbines are shown in U.S. Pat. No. 4,319,865 to Richard, U.S. Pat. No. 4,330,714 to Smith, U.S. Pat. No. 4,350,895 to Cook, U.S. Pat. No. 4,545,729 to Storm, U.S. Pat. No. 4,729,716 to Schmidt, and U.S. Pat. No. 6,664,655 to Vann. Such wind turbines are often of smaller size and utilize sail-like blades rather than complicated blade structures in order to increase the number of wind catching elements, since the weight of the annular rim adds structural problems to the design.
It is important in wind turbines to maintain a relatively steady rotation rate, since this allows for simpler gearbox or other rotation transfer mechanisms. Since wind speed will vary, it is known to provide mechanisms and methods for adjusting the pitch of the blades to account for stronger or weaker winds. This can be accomplished by measuring the wind speed directly or by measuring the rate of rotation of the rotor assembly and then adjusting the blade pitch as required to keep the rotation rate within desired parameters. While this addresses the problem on a macro scale, the known solutions do not take into account localized variations in wind speed striking the rotor assembly at different locations. For example, especially with large diameter wind turbines, the wind speed at the top of the rotor assembly may be significantly greater than the wind speed at the bottom of the rotor assembly. This variation in wind speed also negatively stresses the rotor assembly, which can damage the wind turbine.
It is an object of this invention to provide a system and a method that measures localized wind speed at various points on the wind turbine or the rotor assembly, such that the blade pitch of each individual blade can be more precisely adjusted in response to the actual wind speed being encountered as the blade rotates about the central hub.